Preacher is the best long run out of these guys' work, but I think Millar writes better on shorter stories. He probably produces more crap than Ennis, but some of his minis/small runs are pretty tightly constructed. Red Son, Wolverine: Enemy of the State, Ultimates, Ultimate Fantastic Four...then there's crap like MK Spidey, and just "okay" stuff, like Civil War (which I liked, but it dragged in the middle for about 3 issues). I think I'm gonna have to forsake the epic brilliance of Preacher in the name of quantity.
Of course I haven't read that much Ennis. Preacher, The Boys, and some Punisher, is all. I haven't really read that much Millar, either. Just the aforementioned. No Chosen, Wanted, etc. Weak I know